A secondary battery has high applicability depending on the product group and excellent electric characteristics such as high energy density, and thus not only commonly applied to mobile devices, but also electric vehicles (EV), hybrid electric vehicles (HEV) or the like, driven by an electric power source. Such a secondary battery significantly reduces the use of the fossil fuels and does not generate by-products caused by the use of energy. Therefore, secondary batteries are drawing attention as an eco-friendly alternative energy source with improved energy efficiency.
A secondary battery includes a cathode current collector, an anode current collector, a separator, an active material, a liquid electrolyte, etc., and has a chargeable and dischargeable structure due to the electro-chemical reaction among the components. Meanwhile, since recently a secondary battery is frequently used as an energy storing source and the need of a battery structure having a large capacity is increasing, a secondary battery pack with a multi-module structure having a plurality of the secondary batteries connected to each other in series or in parallel is commonly used.
A secondary battery pack used in a power storage system includes a secondary battery module having a plurality of secondary battery cells aggregated therein and a pack case. In addition to this fundamental structure, a secondary battery pack further includes a Battery Management System (BMS) for monitoring and controlling the status of a secondary battery cell or a secondary battery module by applying an algorithm for controlling power supply to a load, measuring an electric characteristic value such as current, voltage or the like, controlling charge or discharge, controlling equalization of voltage, estimating stage of charge (SOC), etc.
Meanwhile, in order to meet various voltage and capacity requirements, a power storage system may be configured by assembling small-capacity power storage unit racks, each having a plurality of secondary battery packs as described above, in series or in parallel. Recently, since a smart grid is at the center of interest, the need for a large-capacity power storage system storing unused power is increasing to implement an intelligent power network.
A power storage unit rack includes a plurality of secondary battery packs and each secondary battery pack includes a plurality of secondary battery cells or modules therein. A plurality of such power storage unit racks is connected to each other depending on the demanded capacity of a power storage system. In other words, a power storage system includes several tens or thousands of secondary battery cells or modules therein. In order to operate the power storage system, voltage, current, temperature, SOC, or the like of the several tens or thousands of cells or modules should be continuously monitored.
For monitoring the status of each secondary battery cell or module and efficiently controlling the cell or module, correlations of BMSs included in the battery packs are set, so that BMSs included in the battery packs are set to slave BMSs, and a separate BMS capable of controlling the slave BMSs is set to a master BMS to integrally operate and control the power storage system.
Meanwhile, the concept of a power storage system recently tends to broaden beyond a single house or office, namely to a large building, a small district, a city, or a nation, like a smart grid. However, the conventional method described above should possess and manage individual hardware mechanisms or software driving mechanisms, as much as slave BMSs included in a secondary battery pack. Also, as the capacity of a power storage system increases, the conventional method takes more time to allow a master BMS to receive information from each slave BMS and process the information. That is, the conventional method is not sensitive to external changes and fails to achieve energy efficiency, which is an inherent advantage of the smart grids.
Therefore, there is an increasing need for developing a power storage system having a BMS connection structure where BMSs may be installed for each cell, module or pack and integrally managed with efficiency, and its control method.